1. Field of the Invention
The invention relates to ink formulations suitable for use in inkjet printers. More specifically, the invention relates to surface modified pigments for use in ink formulations.
2. Description of the Related Art
Inkjet printing is one of the widely used print methods in commercial products. Ink formulations used in inkjet printing include de-ionized water, a water-soluble organic solvent, and a colorant. The colorant may be a dye or a pigment. However, dyes have poor water-fastness, poor light-fastness, poor ozone stability, and poor thermal stability. This affects the print quality. Accordingly, pigments are used in many ink formulations because they provide better light-fastness, ozone resistance, humidity fastness, resistance to oxidation, and higher thermal stability.
However, pigments are insoluble in aqueous solutions and therefore various methods are employed to improve the dispersibility of the pigments. The methods employed to improve a pigment's dispersibility include surface oxidation and polymer dispersant milling. The surface oxidation method involves generating functional groups, such as carboxy, hydroxy, and sulfonyl, on the surface of the pigment particles. The pigments modified by the surface method are referred to as self-dispersed pigments. An example of a self-dispersed pigment is the surface modified carbon black or self-dispersed carbon black. The ink formulations made from self-dispersed pigments have good optical density but poor adhesion with respect to the print media specifically if they are applied to micro-porous paper. The poor adhesion to the print media results in susceptibility to image quality defects by abrasive forces applied perpendicular or parallel to the plane of the ink on the paper.
The polymer dispersant milling method uses polymeric dispersants that are adsorbed on to the surface of such pigments. The pigments modified by the polymer dispersant milling method are referred to as dispersed pigments. The ink formulations made from dispersed pigments have better resistance to abrasion and adhesion to the print media but poor optical density and slow drying rate specifically on plain paper and photo paper. Further, there may be lot of free polymeric dispersants dissolved in aqueous phase of the ink formulation, which may result in jetting issues such as flooding on or around the nozzle plate of the printhead.
Accordingly, there is a need for an ink formulation that provides good adhesion and optical density on print media especially on micro-porous photo papers. Further, the ink formulation should have good thermal stability.